A typical traction elevator system used in buildings includes a cab that is supported upon a hoist rope. The hoist rope, in turn, is passed over a motor driven drive sheave and the other end of the rope is connected to a counterweight. As the drive sheave is turned, the cab unit and the counterweight unit move vertically along guide rails in opposite directions. The counterweight generally has a weight equal to that of the cab unit plus some additional weight which is a percentage of the cab's rated load capacity.
Safety systems are installed to prevent potentially dangerous over speeding of the cab. To this end, safety units, generally referred to as "safeties", are provided to bring the cab to a controlled halt any time an over speed condition is detected. Safeties generally employ clamps, wedges, rollers or combinations thereof that apply a holding force against the guide rails of the cab unit and/or the counterweight unit to bring the cab to a rapid, safe halt.
In most cases, a governor is used to sense the occurrence of an over speed condition and take some type of responsive action to activate the safeties on both the cab unit and the counterweight unit. As shown in FIG. 1, the governor unit 10 that is in wide use today, includes a pair of governor ropes 11, 13, one of which is connected to the elevator cab 12 and the other of which is attached to the counterweight unit 14. Each governor rope passes over an upper sheave 15, 16 and a lower sheave 17,18 to create a pair of endless loops. Weights 19 and 10 are suspended from the lower sheaves which serve to tension the associated governor rope and maintain the opposed runs of each governor rope in parallel alignment. The governor ropes are connected in a well known manner with safeties 22 associated with each of the units. A governor brake 24 is associated with each governor unit which is adapted to engage the governor rope when the speed of the rope exceeds a given velocity. An over speed condition may be detected by flyweights or electrical sensors which activate the governor brake slowing the governor rope down. This, in turn, trips the unit safeties bringing the elevator to a halt.
This type of prior art governor works extremely well in practice, as evidenced by the safety record of traction elevators. However, as should be evident from the disclosure above, the system requires a separate governor rope for both the cab unit and the counterweight unit. This, in turn, is space consuming, increases installation and maintenance time, and increases the overall cost of the system.